Research using NASA data
is giving new insight into one of the processes causing Greenland's ice sheet
to lose mass. A team of scientists used satellite observations and ice
thickness measurements gathered by NASA's Operation IceBridge to calculate the
rate at which ice flows through Greenland's glaciers into the ocean. The
findings of this research give a clearer picture of how glacier flow affects
the Greenland Ice Sheet and shows that this dynamic process is dominated by a
small number of glaciers.

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Over the past few years,
Operation IceBridge measured the thickness of many of Greenland's glaciers,
which allowed researchers to make a more accurate calculation of ice discharge
rates. In a new study published in the journal Geophysical Research Letters,
researchers calculated ice discharge rates for 178 Greenland glaciers more than
one kilometer (0.62 miles) wide.

Ice sheets grow when snow
accumulates and is compacted into ice. They lose mass when ice and snow at the
surface melts and runs off and when glaciers at the coast discharge ice into
the ocean. The difference between yearly snowfall on an ice sheet and the sum
of melting and discharge is called a mass budget. When these factors are equal,
the mass budget is balanced, but for years the Greenland Ice Sheet has had a
negative mass budget, meaning the ice sheet is losing mass overall.

For years the processes of
surface melt and glacier discharge were roughly equal in size, but around 2006
surface melt increased and now exceeds iceberg production. In recent years,
computer model projections have shown an increasing dominance of surface melt,
but a limited amount of glacier thickness data made pinpointing a figure for
ice discharge difficult.

Ice discharge is
controlled by three major factors: ice thickness, glacier valley shape and ice
velocity. Researchers used data from IceBridge's ice-penetrating radar — the
Multichannel Coherent Radar Depth Sounder, or MCoRDS, which is operated by the
Center for Remote Sensing of Ice Sheets at the University of Kansas, Lawrence,
Kan. — to determine ice thickness and sub-glacial terrain, and images from
satellite sources such as Landsat and Terra to calculate velocity. The team
used several years of observations to ensure accuracy. "Glacier discharge
may vary considerably between years," said Ellyn Enderlin, glaciologist at
the University of Maine, Orono, Maine and the study's lead author. "Annual
changes in speed and thickness must be taken into account."